1. Technical Field
The present invention relates to an optical fiber used for optical communication, and in particular to an optical fiber with low transmission loss of propagated light even when curved.
2. Related Art
When communicating through an access system such as FTTH that uses optical fiber for communication between a station and user homes, it is possible that the optical fiber will be curved to have a curvature between several millimeters and tens of millimeters. Therefore, for optical fiber used for wiring in the home of FTTH, low transmission loss relative to the curvature is desired. Since a long distance backbone cable is arranged in a location that is not easily affected by outside forces, e.g. in an underground duct, the bending force placed on these optical fibers is expected to result from no more than winding the optical fiber (up to 100 times) with a diameter of 60 mm within a terminal device. In contrast to this, the optical fiber inside and outside a home can bend, is light-weight, and is formed as a relatively thin cord, e.g. with a diameter of several millimeters, and therefore the optical fiber is easily affected by outside forces and often experiences a curvature radius of tens of millimeters or less.
The optical fiber propagates the signal light through the core of the optical fiber, and therefore transmission is still possible when the optical fiber is in a curved state. However, when the curvature radius is smaller, the ratio of light that leaks out of the core without being propagated increases exponentially, resulting in transmission loss. This is referred to as “bending loss.” Increasing the refractive index of the core and focusing more of the light in the core is effective for reducing the bending loss, and this effect can be improved by lowering the mode field diameter (MFD). Therefore, conventional optical fiber with an MFD of approximately 6 to 8 μm is often used, in which case the bending loss when the optical fiber is wound around a mandrel (cylinder) with a diameter of 20 mm is no greater than 0.5 dB/turn for a wavelength of 1550 nm.
The MFD of an optical fiber in compliance with the ITU-TG.652 standard, which is commonly used for optical communication in a long-distance system, it approximately 8 to 10 μm, and therefore there is a problem that, when this optical fiber is connected to the optical fiber with lower MFD, the difference in MFD causes connection loss. Therefore, it is preferably that the optical fiber of an access-system have an MFD of approximately 8 to 10 μm. A trench-type optical fiber that can lower the bending loss while employing a design with high MFD is described in U.S. Pat. No. 4,852,968 and in the technical document “Optical Fiber Comprising a Refractive Index Trench” by William A. Reed. This technique has been known for a long time, but these excellent bending loss characteristics have recently attracted a lot of attention.
In the case of a quartz glass optical fiber, the core is doped with germanium to increase the refractive index and the trench portion is doped with fluorine to decrease the refractive index. Inner and outer cladding is formed by pure quartz or is doped with only a small amount of fluorine or germanium, thereby bringing the refractive index of the cladding near that of quartz.
When manufacturing a normal optical fiber base material using VAD, (1) a core (first core) and inner cladding (second core) are formed, to create a core/cladding glass intermediate body (intermediate body). Next, (2) the trench portion (third core) is formed. The trench portion can be formed by carefully depositing glass soot particles on the outside of the intermediate body, and thermally processing the resulting member in an atmosphere of a gas containing fluorine. Finally, (3) the outer cladding is formed.
At this time, the intermediate body on which the glass soot particles have been deposited is simultaneously supplied with an inert gas such as helium and a gas containing fluorine, such as SiF4 or CF4, as the atmospheric gas and heated to approximately 1300° C., thereby doping the trench portion with fluorine. It is known that the doping concentration of the fluorine increases in proportion to approximately the 0.25 power of the pressure of the gas containing fluorine in the atmospheric gas, and the pressure of the gas containing fluorine must be increased when the doping concentration is higher. For example, in order to achieve a relative refractive index difference (Δ) of −0.6% for the trench portion, SiF4 with pressure of approximately 0.7 atm is necessary, and this creates the problem that a large amount of gas containing fluorine is consumed.
Furthermore, concerning the refractive index distribution of the trench, the inner portion of the trench easily propagates higher modes of light, thereby creating a trend of increasing the cutoff wavelength. Therefore, a precise design for decreasing the core diameter is necessary. However, in the case of a design that decreases the core diameter, there is a problem that the zero-dispersion wavelength is shifted to a longer wavelength. The zero-dispersion wavelength is the wavelength at which the wavelength dispersion is zero, and when the absolute value of the wavelength scattering is large, the long-distance transmission quality decreases due to widening of the optical signal pulse, for example.
Not only is optical fiber used independently in long-distance systems and access systems, but there is also commercial demand for adopting optical fiber used in an access system in a long distance system as well, without alteration. In this case, the zero-dispersion wavelength is preferably approximately 1.31 μm, which is the optical transmission wavelength of a normal single mode optical fiber, and more preferably in a range from 1.300 to 1.324 μm.
In light of the above, it is an objective of the present invention to provide an optical fiber that exhibits a trench-type refractive index distribution with excellent bending characteristics while expanding the MFD in a range of approximately 8 to 10 μm, to provide an optical fiber that has excellent bending characteristics and low consumption of the gas containing fluorine used when doping the trench portion with fluorine, and to provide an optical fiber in which the zero-dispersion wavelength is designed to be within a range of 1300 to 1324 nm, which is equivalent to the range of a single mode optical fiber.